One known flexible circuit assembly with photonics includes a fiber optic transmitter die, such as a vertical cavity surface emitting laser (VCSEL), and a fiber optic receiver die, such as a PIN diode array (PDA), mounted on a die carrier. The die carrier is mounted on the flexible circuit substrate so that the transmitter die and the receiver die optically transmit or receive optical signals through the material of the flex circuit substrate. Examples of this known type of flexible circuit assembly are disclosed in U.S. 2011/0052124 and U.S. 2011/0058818. However, optical transmission through the flex circuit substrate material can cause attenuation of the optical signals.
In the known flexible circuit assembly in the preceding paragraph, a flip chip assembly technique has been used to electrically attach the photonics to the electrical circuitry of the flexible circuit. The transmitter die and receiver die are bumped and then “flip chipped” or scrubbed into the flexible circuit to electrically connect to the electric circuitry of the flexible circuit. In this technique, both die need to be substantially coplanar or at the same height in order to achieve adequate electrical connection. However, due to die thickness variance, this has proved to be problematic leading to an unacceptably high number of rejects due to inadequate electrical connection.
In addition, an optical epoxy under-fill is utilized between the die and the flex circuit. However, the epoxy under-fill creates problems because the epoxy tends to flow into the optical connector pin holes.